As is known, communication links which utilize packet data formats, such as 10 Mbps Ethernet, and baseband binary encoding formats such as Manchester or delay modulation (also known as Miller coding) commonly employ receivers with amplitude limiting. Direct detection optical receivers with ac-coupled baseband limiting are commonly used for wireless data communication due to their low cost and simplicity. However, full-swing noise is present at the output of these receivers between data packet transmissions. In LAN (local area network) systems using a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) protocol, such as Ethernet, interpacket noise may be misinterpreted as a collision, i.e. multiple sources transmitting on the LAN at the same time. To avoid misinterpretation of noise, it is necessary to distinguish between signal and noise. This is typically accomplished by a signal presence indicator (SPI) which squelches the output of the receiver between packet transmissions.
Various types of SPIs are known in the art. For example, one SPI commonly used in direct detection optical fiber receivers compares the received signal amplitude to a fixed threshold. While this approach works in many instances, it is not suitable for wireless optical receivers since an increase in the intensity of ambient background radiation due to daylight, incandescent lamps, or nearby optical transmitters in the same system may produce a false indication of signal presence. To avoid this problem, it is necessary to continuously monitor the background noise level and adjust the threshold level to ensure proper operation without false indications. Since the level of the background noise varies widely, a logarithmic receiver would be necessary to ensure adequate dynamic range, which is cost prohibitive.
Therefore, a need exists for a signal presence indicator for direct-detection wireless optical data amplitude limiting receivers.